1. Field of the Invention
The invention relates to semiconductor processing techniques, more particularly, to controlling constituents of a film introduced onto a substrate.
2. Description of Related Art
In the formation of modern integrated circuit devices, many constituents are introduced to a substrate such as a wafer to form films. Typical films include dielectric material films, such as transistor gate oxide or interconnect isolation films, as well as conductive material or semiconducting material films. Interconnect metal films and polysilicon electrode films, respectively, are examples of conductive and semiconducting material films.
In addition to the above-noted material films, other constituents are often introduced onto a substrate such as a wafer or a structure on a substrate to change the chemical or conductive properties of the substrate or the structure. Examples of this type of constituent introduction includes, for example, the deposition of a refractory metal onto an electrode or junction to form a silicide and the deposition of germane onto a substrate to form a silicon germanium junction in a bipolar transistor. The introduction of constituents onto a substrate or structure on a substrate such as described is referred to herein as a subset of film formation.
One way to enhance the performance of integrated circuit devices is to improve control of the introduction of the constituents, such as improved control of the introduction of process gas species in deposition introduction. Many wafer process chambers, including the EPI Centura system, commercially available from Applied Materials, Inc. of Sunnyvale, Calif., utilize mass flow controllers to introduce process gas species. In general, a mass flow controller functions by permitting a desired flow rate of a gas species based on an input signal to the mass flow controller demanding the flow rate. The concentration profile of a species constituent within a film deposited on a substrate is then a function of the mass flow rate of species introduced. In general, the relationship between a species concentration profile or gradient introduced into or onto a substrate, for example a wafer, and the mass flow rate of the species introduced is not necessarily linear.
In general, mass flow controllers are used to either supply a constant flow rate or a variable flow rate from a first flow set point to a second flow set point over a period of time. One common flow ramp between a first set point and a second set point is a linear flow ramp. A linear ramp, however, does not necessarily produce a desired concentration profile, e.g., a linear profile, of the species in the introduced film. In the example of a species of germane (GeH4) introduced to form a silicon-germanium film, a graded film is desirable in many situations. The desired graded profile in the film, for example germanium concentration profile, may be linear or non-linear. The method to control a mass flow controller to precisely control the amount of flow and produce the desired germanium concentration profile in the junction, whether it is linear or non-linear, is of significant importance. In commercial use, targeting a desired concentration profile, for example a linear profile, has generally not proved possible through a linearly increasing or decreasing constant flow introduction of the germanium species by a mass flow controller.
What is needed is a way to control the introduction of a species to form a film having a desired concentration profile of the species in the film. The ability to quantitatively control the introduction of a species through a mass flow controller to form a film with a specific film thickness is also desirable.